I agree with LoveDoctor. It would change things. If you like this friendship the way it is, leave it that way.
My wife and I are so new to this that I think I can look back well less than a year and see how we would have reacted if one of our friends had "come out" to us. This whole decision to swing is not made instantaneously, and the foundation that allows it to be made is laid over time. Think of the years of societal programming that makes us think that sex is dirty or harmful and that sharing your spouse with someone is the greatest sin ever. Bit by bit over time we begin to ask why. If you started with only the golden rule (of any religion) and built a system of morals to live by from it, you would end up with a system that looks remarkedly like that of the major religions. We can extrapolate from the golden rule to get rules not to steal, injure, kill, lie, and pretty much everything else that modern religions hand us. What we would not come up with, however, is a rule against swinging. We would certainly come up with a rule against cheating, but swinging requires concensus from all and results in only increased pleasure and happiness. And yet society has a strict taboo against sharing sex with other couples. It is only, I think, when people start to internalize their own system of morals that they begin to see that society has almost everything right, but not the prohibition against swinging. All of us that swing have passed that point and operate on an internal system of values that would survive even if we found out tomorrow that all religions are false. This gives us a great advantage over people that have external systems of value that have not yet been internalized.
Regardless of how they act, it is difficult to be sure that your friends have reached the point where they ask why when presented with a "moral" that doesn't violate the golden rule (like swinging). Think back to the time before swinging ever entered your mind. How would you react if your friends said that they were swingers? Would it change things? Probably.
I hope you don't think I am just arrogant, but I think that we swingers (as a group) are "further down the road" in terms of filtering out the societal norms that cannot be derived from the golden rule. We have some great friends, too, but since we can't know how far they are down the road, we keep our lifestyle to ourselves. But that is just us, and that is just now. Maybe with more experience we will feel more comfortable and won't be such cowards.
However, I can certainly understand the desire to tell them in an effort to increase their own happiness. Be careful that it doesn't backfire.